How thick is solid wall insulation?

Thickness is really a question about performance versus space: more insulation means a lower U-value but a deeper build-up. Here is how the numbers work for both methods.

Thickness and U-value go together

Insulation thickness only makes sense alongside a target U-value — the measure of how much heat passes through the wall, where a lower number is better. An uninsulated solid brick wall is often around 2.0 W/m²K or worse; UK Building Regulations for renovating a wall typically expect you to reach roughly 0.30 W/m²K where it is technically and economically feasible. Reaching that target needs a certain thermal resistance, and how much physical thickness that takes depends entirely on the material's conductivity. A high-performance rigid board reaches the target with less depth; a more breathable, lower-performance material needs more. So 'how thick' is answered by 'which material, and what U-value'.

Internal versus external thickness

Internal (IWI): because every millimetre comes off your room, IWI is often specified with higher-performance boards to keep the build-up down — commonly around 60–120mm in total once the insulated board and skim are in. Thinner 'space-saving' systems exist but accept a higher (worse) U-value. External (EWI): there is no internal space penalty, so EWI can use a thicker layer for a better result — commonly around 90–150mm including the reinforced render. EWI thickness is limited instead by the depth the roof overhang, window reveals and door thresholds can accommodate without major alterations.

Indicative ranges for guidance; actual figures depend on the product and U-value target. Sources: Energy Saving Trust solid wall insulation guidance.

The performance-versus-space trade-off

With internal insulation there is a genuine tension: a thicker layer gives a lower U-value and a warmer, more comfortable room, but takes more floor space and makes window reveals deeper. A thinner layer keeps the room size but insulates less and can leave the wall surface cooler, raising condensation risk if not detailed well. Choosing the material is the lever — a high-performance board buys you a good U-value in less depth, while a breathable wood-fibre system needs more depth but suits older walls better. External insulation largely avoids the space trade-off, which is one of its practical advantages.

Practical details thickness affects

Whatever thickness you choose, it has knock-on effects to plan for. Internally, deeper insulation means deeper window reveals, repositioned radiators and skirting, and slightly smaller rooms — worth checking against fitted furniture. Externally, the added depth changes how far the insulation projects past windows and at the roof eaves, sometimes needing the roof verge or guttering extended and the window reveals re-detailed to shed water. A competent installer designs the thickness around both the U-value target and these junctions, ideally as part of a PAS 2035 assessment, so the finished wall performs and stays weathertight.

How thickness is decided in practice

Rather than picking a number off the shelf, a good installer works backwards from the result. They start with the target U-value for the job, choose a material suited to the wall (high-performance board where space is tight, breathable wood-fibre on an older wall), and then calculate the thickness needed to hit the target with that material. The same target therefore produces different thicknesses depending on the product, which is why two valid quotes can specify quite different depths. If you are comparing options, ask for the U-value each system achieves rather than just the millimetres, because the millimetres alone do not tell you how warm the wall will be.

There are also sensible limits. Internally, beyond a certain depth the lost room space outweighs the extra performance, so there is a practical ceiling on how thick it is worth going. Externally, the limit is usually set by how far the insulation can project at the eaves, window reveals and door thresholds before those details need rebuilding. Within those limits, the aim is simply enough thickness to reach the target U-value in a material that suits the wall — not the thinnest system that fits, and not the thickest the wall could physically carry.

Planning for the depth before work starts

Because the build-up has a real depth, it pays to map its effects before committing. Internally, walk each room and note what sits against the external walls — fitted wardrobes, kitchen units, boxed-in pipework, radiators and electrical sockets — since all of these interact with a wall that is about to move 60–120mm into the room. Deeper window reveals change how curtains, blinds and shutters fit, and a door near a treated wall may need its frame or swing adjusted. None of this is a reason against insulating, but discovering it on paper is far cheaper than discovering it mid-job.

Externally, the things to check are at the edges of the building: whether the roof overhang is deep enough to cover a thicker wall or needs extending, how the insulation will turn around window and door reveals without creating cold bridges, and whether gutters, downpipes and sills need moving out to suit the new face. A good installer surveys all of this up front and designs the thickness around it, so the chosen depth is one the building can actually accommodate — which is the difference between a thickness that looks good in a spec and one that works on the wall.

Why the same wall can take different thicknesses

Two identical-looking solid walls can end up with very different insulation depths, and that is normal. The drivers are the material's conductivity, the U-value being targeted, and the space available. A high-performance rigid board might reach the target in well under 100mm, while a breathable wood-fibre system specified for an older wall might need noticeably more for the same result — yet the wood-fibre is the right answer where the wall must breathe. Neither is 'too thick' or 'too thin'; each is the correct depth for its material and purpose.

This is why the thickness figure on its own is a poor way to compare systems. A thinner build-up is not automatically better value, because it may be insulating less, and a thicker one is not automatically over-specified, because it may simply be a breathable material doing the same job. When weighing options, anchor the comparison on three things together — the U-value achieved, the material and its suitability for the wall, and the space it costs you — and the right thickness for your home becomes a sensible outcome of those, rather than a number to chase up or down.

Frequently asked questions

Sources & further reading

• Energy Saving Trust — solid wall insulation
• GOV.UK — building regulations and energy efficiency

Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.